Abstract: Isotopes of an element are separated using a method in which a gaseous compound containing said element is brought into a condition in which the molecules of the compound are in the ground vibrational state, and the molecules containing the pre-determined isotope are selectively excited by means of a source of radiation. The selectively excited molecules are further excited, and/or directly dissociated, using at least a second source of radiation, including at least two laser beams which are converted using so-called "four-wave mixing" in a Raman cell. One of the laser beams is a high-energy beam that is a train of very short pulses.